Airport Express network

This is probably really simple to solve but I have tied myself in knots trying to sort it.
I have a Powerbook G4 with Airport connected to a Buffalo wireless router. I also have a mac Mini without any in-built wireless connection.
I bought an Airport Express to connect my Mac Mini to it the internet. I just can't seem to get the mini connected.
The AE is set up so my G4 can see it but the AE can't connect to the internet - it doesn't seem to be talking to the Buffalo router. When I connect the AE to the Mini, the Mini can see it but again I can't connect to the internet.
The only way I can connect is to use an ethernet cable from the G4 to the mini.
Any ideas how I can get them to connect?